High tensile strength, refractory steel having excellent weldability and gas cuttability and method for producing same

ABSTRACT

Accordingly to an exemplary embodiment of the present invention, a high tensile strength, refractory steel can be provided which comprises, in mass %, approximately C: 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, and the balance consisting of iron and unavoidable impurities. For example, a weld crack sensitive composition P CM  can be defined by the following equation may be about 0.25% or less: 
       P CM =C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B. 
     An area fraction of polygonal ferrite or pseudo polygonal ferrite in a ¼ thick position in the plate thickness direction of the steel plate of the final rolling product is about 10% or less.

CROSS REFERENCE TO RELATED APPLICATION(S)

The present application is a national phase application of InternationalApplication PCT/JP2006/304127 filed on Mar. 3, 2006 and published asInternational Publication WO 2006/093282 on Sep. 8, 2006. Thisapplication claims priority from the International Application pursuantto 35 U.S.C. § 365. .The present application also claims priority fromJapanese Patent Application No. 2005-060601 filed on Mar. 4, 2005 under35 U.S.C. § 119. The disclosures of these applications are incorporatedherein in their entireties.

FIELD OF THE INVENTION

The present invention relates to a high tensile strength, refractorysteel having excellent weldability and gas cuttability, and a method ofproducing the same.

BACKGROUND INFORMATION

As a refractory steel for architectural construction, intended toprovide a high temperature strength at a time of fire or the like, arefractory steel obtained by hot rolling a billet or slab has beendescribed (see, for example, Japanese Unexamined Patent Application ,First Publication No. H2-77523).

This refractory steel generally belongs to so called 400 MPa class steelor 490 MPa class steel, and can include several examples of so called590 MPa class steel having an yield strength of 440 MPa (45 kgf/mm²) ormore.

On the other hand, as a refractory steel corresponding to the 590 MPaclass steel, steel containing Mo of 0.7% or more has been described(see, e.g., Japanese Unexamined Patent Application, First PublicationNo.2002-12939).

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

In “rolled steel for architectural construction” JIS G 3136 according toJapanese Industrial Standard and “high performance 590 N/mm² steel forarchitectural construction (SA440B,C)” qualified by the Minister ofLand, Infrastructure and Transport of Japan as examples of constructionsteel, the plate thickness can be regulated up to 100 mm. However, inthe conventional types of refractory steel mainly constituting 400 MPaclass steel and 490 MPa class steel, the plate thickness of the 590 MPaclass steel can be at most, e.g., 40 mm, and thicker steel may not besupplied.

Recently, there has been an increasing demand for steel having a yieldstrength of 440 MPa or more and belonging to a class not lower thanso-called 590 MPa class. Such a steel can be subjected to thermalrefining. After the thermal refining, a hot rolled steel can have ametallic texture mainly composed of, e.g., polygonal ferrite or pseudopolygonal ferrite having low strength. Therefore, even when a thicksteel plate having a thickness of about 100 mm is produced by hotrolling, it may be difficult to ensure the strength of the steel stablyby a technical control.

On the other hand, a refractory steel corresponding to 590 MPa classsteel may have a steel composition containing Mo of 0.7% or more, likelyresulting in inferior cuttability by gas-cutting and high productioncost. In addition, although weld crack sensitive composition (P_(CM))can be controlled in this refractory steel, Mo generally enhances thehardenability of the steel. From the view point of weldability, it maybe preferable that the Mo content can be controlled to a low level.

Exemplary embodiments of the present invention may take the above issuesinto consideration. Thus, one of the objects of the present invention isto provide a high tensile strength, refractory steel and method ofproducing the same which has an excellent weldability and cuttability bygas cutting so as to allow mass production at low cost of a high tensilestrength steel having a yield strength of 440 MPa or more and possiblyhaving sufficient high temperature strength under a high temperatureenvironment, such as, e.g., a fire.

Research for the above-described exemplary issue has been performed.Based on such research, it has been determined that by compound additionof Nb while suppressing the content of Mo, it is possible to stablyprovide and/or ensure the high temperature strength of a high tensilestrength steel having a yield strength of 440 MPa or more. Bysuppressing the Mo content in the steel, it is possible to suppress thedeterioration of weldability and gas cuttability of the steel to aminimum level. At the same time, by controlling PCM and the contents ofrespective alloy elements such as C, Si, Mn, and by further limiting themicrostructure of the steel and manufacturing conditions for themicrostructure, it is possible to obtain consistently complex propertiesincluding excellent high temperature strength, weldability, and gascuttability.

According to one exemplary embodiment of the present invention, a hightensile strength, refractory steel having excellent weldability and gascuttability can be provided. For example, this steel can include, inmass %, approximately, : 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo:0.30% or more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to0.0060%, and the balance consisting of iron and unavoidable impurities.A weld crack sensitive composition PCM of the steel is 0.25% or less,and can be defined by the following equation:

P_(CM)=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B,

and the area fraction of polygonal ferrite or pseudo polygonal ferritein a ¼ thick position in the plate thickness direction of a steel plateof the final rolling product can be 10% or less.

This exemplary embodiment of the steel can further include, in mass %,approximately, Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, and one or two ormore selected from Cr: 0.05 to 1.0%, V: 0.01 to 0.06%, B: 0.0002 to0.0030%, Ti: 0.005to 0.025%, Mg: 0.0002 to 0.0050%, and the Ni contentmay be at least half of the Cu content. Such exemplary steel can furthercomprise, in mass %, approximately, one or two selected from Ca: 0.0005to 0.0040% and REM: 0.0005 to 0.0100%. The yield strength of the steelcan be 440 MPa or more.

In accordance with another exemplary embodiment of the presentinvention, a method for manufacturing a high tensile strength,refractory steel having excellent weldability and gas cuttability, canbe provided. For example, according to this exemplary method, a steelmember in a form of billet or slab can be heated at a temperature ofabout 1100 to 1300° C., with the billet and/or slab having the steelcomposition that includes, in mass %, approximately, : 0.04 to 0.14%,Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% orless, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%, Al:0.060% or less, N: 0.0010 to 0.0060%, and the balance consisting of ironand unavoidable impurities. The steel member is rolled at a temperatureof 800 to 950° C. The steel member is directly quenched from atemperature not lower than a higher one selected from about 750° C. or atemperature about 150° C. lower than a temperature at a time ofcompleting the rolling. The steel member is tempered at a temperaturenot higher than Ac₁(e.g., a temperature at which generation of austenitestarts at a time of heating).

According to still another exemplary embodiment of the presentinvention, a method for manufacturing high tensile strength, refractorysteel having excellent weldability and gas cuttability can be provided.For example, a steel member can be hot-rolled in a form of billet orslab, which has the steel composition that includes, in mass %,approximately, : 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P:0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or moreand less than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, and thebalance consisting of iron and unavoidable impurities. The steel memberis self-cooled, and quenched after reheating the steel member at atemperature of about 900 to 950° C. The steel member is tempered at atemperature not higher than Ac₁.

According to a further exemplary embodiment of the present invention, ahigh tensile strength, refractory steel having excellent weldability andgas cuttability can be provided which may have a weld crack sensitivecomposition PCM of about 0.25% or less and the balance consisting ofiron and unavoidable impurities. For example, PCM can be defined by thefollowing equation:P_(CM)=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B. Further, an areafraction of polygonal ferrite or pseudo polygonal ferrite in a ¼ thickposition in the plate thickness direction of a steel plate of the finalrolling product can be about 10% or less. According to such a hightensile strength, refractory steel, it may be possible to perform massproduction at low cost of high tensile strength steel having a yieldstrength of about 440 MPa or more, having excellent weldability and gascuttability, and having sufficient high temperature strength under ahigh temperature environment such as a fire.

A construction steel for architectural construction according to yetanother exemplary embodiment of the present invention can be provided,composed of, e.g., the high tensile strength, refractory steel which canbe applied as general construction steel for various applicationsincluding civil engineering, marine structures, ships and vessels,various storage tanks, industrial facilities such as plate-mills or thelike. Since the exemplary embodiment of the high tensile strength,refractory steel according to the present invention has sufficient hightemperature strength even under a severe environment, for example, at atime of fire, in which the steel is exposed to high temperatureconditions, it may be possible to further enhance the safety of weldconstructions.

In accordance with yet a further exemplary embodiment of a method formanufacturing high tensile strength, refractory steel having excellentweldability and gas cuttability according to the present invention, asteel member in a form of billet or slab having the steel compositioncan be heated at a temperature of about 1100 to 1300° C., and rolled ata temperature of about 800 to 950° C. After that, the steel member maybe directly quenched from a temperature of not lower than a higher oneselected from about 750° C. or a temperature about 150° C. lower than atemperature at a time of completing the rolling, and tempered at atemperature not higher than Ac₁. Therefore, it is possible to performmass production at low cost of a high tensile strength steel having ayield strength of 440 MPa or more and having excellent weldability andgas cuttability and sufficient high temperature strength under a hightemperature environment such as a fire, where the steel is exposed tohigh temperature conditions.

In accordance with still another aspect of a method for manufacturinghigh tensile strength, refractory steel having excellent weldability andgas cuttability according to the present invention, after hot-rolling asteel member in a form of billet or slab having the steel composition asdescribed herein above, the exemplary steel member can be self-cooled,and quenched after being reheated at a temperature of about 900 to 950°C., and tempered at a temperature of not higher than Ac₁. Therefore, itis possible to perform mass production at low cost of a high tensilestrength steel having a yield strength of about 440 MPa or more andhaving excellent weldability and gas cuttability and sufficient hightemperature strength under a high temperature environment such as afire, where the steel is exposed to high temperature conditions.

These and other objects, features and advantages of the presentinvention will become apparent upon reading the following detaileddescription of embodiments of the invention, when taken in conjunctionwith the appended claims.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF INVENTION

An exemplary embodiment of high tensile strength, refractory steelhaving excellent weldability and gas cuttability according to thepresent invention and the method for manufacturing the same aredescribed herein.

It should be understood that the exemplary embodiment of the presentinvention described herein are not limited to the description, and canbe used and/or applicable in various other ways and for variousapplications.

For example, an exemplary embodiment of a high tensile strength steelaccording to the present invention can includes, in mass %,approximately, C: 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%,P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% ormore and less than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, andthe balance consisting of iron and unavoidable impurities. A weld cracksensitive composition PCM can be defined by the following exemplaryequation can be 0.25% or less:

P_(CM)=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B,

and an area fraction of polygonal ferrite or pseudo polygonal ferrite ina ¼ thick position in the plate thickness direction of a steel plate ofthe final rolling product can be about 10% or less.

Further, it is possible to limit the composition of the high tensilestrength, refractory steel. For example, C likely has an influence onthe property of the steel. The preferably minimum value of approximately0.04% can be a minimum content so as to ensure the strength, and may beused to reduce or inhibit the over softening than necessary of aheat-affected portion such as a weld portion. On the other hand, toohigh a C content may enhance the hardenability of the steel to anunnecessary level, and can have a negative influence on the strength,toughness balance, and weldability as intrinsic properties of the steel.Therefore, the upper limit of the C content may be set to be about0.14%.

Si can have an influence on cleanability, weldability, and weld-portiontoughness of the steel. Therefore, it may be used to control its upperlimit value. Therefore, the Si content can be set to be about 0.50% orless. Si may also have an effect of deoxidizing of the steel. However,deoxidization of the steel can be performed by Ti or Al. Therefore,where weldability and weld-portion toughness are preferably used, it maynot be necessary to add Si to the steel.

Mn can be an important element for ensuring strength and toughness ofthe steel, and its minimum content may be, e.g., about 0.50%. On theother hand, too high a Mn content can enhance the hardenability of thesteel, deteriorate weldability and toughness of weld-heat affectedportion of the steel, as well as enhance the segregation of the centralportion of the slab during continuous casting process. Therefore, theupper limit of the Mn can be set to be about 2.00%.

P can constitute an impurity in the steel of an exemplary embodiment ofthe present invention. By reducing the P content, grain boundarydeformation in the weld heat-affected portion may be reduced. Therefore,it may be preferable to control the P content to as low as possible.Therefore, so as not to deteriorate low temperature toughness of thebase metal and weld heat-affected portion, the upper limit of the Pcontent can be set to be about 0.020%.

As with P, S may constitute an impurity in an exemplary embodiment ofthe steel of the present invention. In order to ensure low temperaturetoughness of the steel, it may be preferable to control the S content toas low as possible. Therefore, so as not to deteriorate low temperaturetoughness of the base metal and weld heat-affected portion, the upperlimit of the S content can be set to be about 0.010%.

Nb is an element which can play a role in an exemplary embodiment of thepresent invention where Mo content can be depressed as far as possible.Firstly, as a general effect, Nb can be an important element forelevating the recrystallization temperature of austenite, and exertingthe effect of controlled rolling at a time of hot-rolling. In order torealize such effects, it may be preferable for the steel to contain Nbof at least about 0.01%.

Nb may also contribute to grain refining of heated austenite at a timeof reheating preceding the rolling and has an effect of enhancingstrength of the steel by precipitation hardening. In addition, bycomposite addition with Mo, Nb can contribute to the high temperaturestrength of the steel. However, too high a Nb contents may result in adeterioration of toughness of the weld portion. Therefore, so as not togenerate the deterioration of toughness of the weld portion, the upperlimit of the Nb content can be set to be about 0.005%.

Mo can be an important element for ensuring high temperature toughnessof the steel.

In order for the exemplary embodiment of the steel to have sufficienthigh temperature strength under an environment, for example, at a timeof fire, where the steel is exposed to high temperature conditions, itmay be preferable for the steel to contain Mo of about 0.30% or more. Onthe other hand, too high of a Mo content can deteriorate weldability andgas cuttability of the steel. Therefore, the upper limit of the steelcan be set to be less than 0.70%.

Al is a deoxidizing element. However, deoxidization of the steel can besufficiently performed, e.g., solely by Si or by Ti. Therefore, thelower limit of Al content may not be set according to an exemplaryembodiment of the present invention. On the other hand, too high an Alcontent may impair cleanability of the steel, deteriorate toughness ofthe base metal, and deteriorate toughness of the weld-heat affectedportion. Therefore, the upper limit of Al content may be set to be about0.060%.

N can be contained as an unavoidable impurity in the steel. By bondingwith the above-described Nb, N can form carbonitride and enhance thestrength of the steel. In addition, where the below-mentioned Ti isadded, N can enhance strength of the steel by forming TiN. In order toobtain such effects, it may be preferable for the steel to contain N ofat least about 0.0010%.

On the other hand, an increased N content has an adverse effect ontoughness of the weld heat-affected portion and weldability. Therefore,the upper limit of N content can be set to be about 0.0060%.

In addition to the above-described exemplary composition, the hightensile strength steel according to the exemplary embodiment of thepresent invention can further include, in mass %, about Ni: 0.05 to1.0%, Cu: 0.05 to 1.0%, and one or two or more selected from Cr: 0.05 to1.0%, V: 0.01 to 0.06%, B: 0.0002 to 0.0030%, Ti: 0.005to 0.025%, Mg:0.0002 to 0.0050%, wherein the Ni content may be at least half of the Cucontent.

One of the reason for adding these elements to the above-describedexemplary basal composition can be to improve the properties such asstrength and toughness of the steel without impairing excellentcharacteristics of the steel according to the exemplary embodiment ofthe present invention. Therefore, loadings of the exemplary elements maybe restricted.

Where too much loading is avoided, Ni can improve strength and toughnessof the steel without having a negative influence on weldability andtoughness of the weld heat-affected portion of the steel. To realizesuch exemplary effects, it may be preferable for the steel to contain Niof at least about 0.05% or more. On the other hand, too high a loadingof Ni may elevate the price of the steel and also has an undesirableeffect on weldability. Therefore, the upper limit of Ni content can beset to be about 1.0%.

Where Cu is added, in order to prevent the occurrence of Cu-cracksduring the hot-rolling, it is necessary to control the Ni content to benot lower than ½ of the Cu content while controlling the Ni content inthe above-described range.

Cu can show nearly similar functions and effects as those of Ni.However, in addition to deteriorating the weldability, too high aloading of Cu may cause Cu-cracks to occur at the time of hot-rollingand makes it difficult to produce the exemplary steel. Therefore, theupper limit of the Cu content may be set to be about 1.0%. On the otherhand, in order to obtain a substantial effect, it may be preferable forthe steel to contain a minimum amount of Cu. Therefore, the lower limitof the Cu content can be set to be about 0.05%.

Cr may improve strength and toughness of the base metal. However, toohigh a Cr content can deteriorate the toughness of the base metal andweld portion and weldability. Therefore, the upper limit of the Crcontent may be set to be about 1.0%. On the other hand, in order toobtain a substantial effect, it may be preferable for the steel tocontain a minimum amount of Cr. Therefore, the lower limit of the Crcontent can be set to be about 0.05%.

The above-described Ni, Cu and Cr can be effective for improving weatherresistance as well as for improving strength and toughness of the basemetal. For this purpose, it can be preferable for the steel to containthese elements while controlling their amounts in a range not impairingweldability.

V has similar effects as Nb. However, its effect may be less than thatof Nb. V may have an influence on hardenability and contributes toimprovement of high temperature strength.

In order to realize the same effect as Nb, it is necessary for the steelto contain at least 0.01% V. On the other hand, where the steel containsexcessive V, toughness of the weld portion may be deteriorated.Therefore, so as not to deteriorate the toughness of the weld portion,the upper limit of the V content can be set to be about 0.06%.

B can be segregated in the grain boundary of austenite, may depressoccurrence of ferrite and thereby improves hardenability and strength ofthe steel. In order to realize such effects, it may be preferable forthe steel to contain at least about 0.0002% of B. However, where toomuch B is contained, hardenability-improving effect is saturated, andthere is a possibility of occurrence of B precipitates possibly havingan adverse effect on the toughness of the exemplary steel. Therefore,the upper limit of the B content can be set to be about 0.003%.

When being used as steel for a tank or the like, there is a possibilityof stress corrosion cracking of the exemplary steel. In such a case,reduction of hardness of the base metal and weld heat-affected portionare often important. For example, in order to prevent sulfide stresscorrosion cracking (SSC), hardness of HRC≦22 (HV≦248) may be important.In such a case, it may be not as preferable to add B which enhanceshardenability.

Where high toughness is preferred in the base metal and in the weldportion, it is preferable to load Ti in the exemplary steel. Where theAl content is low, for example, where the Al content is 0.03% or less,Ti can be bonded with O and may form precipitates mainly composed ofTi₂O₃ likely acting as nuclei of generation of ferrite by transgranulartransformation and improving toughness of the weld portion. In addition,Ti can be effective by being bonded with N and forms TiN constitutingfine precipitates in the slab, thereby depressing coarsening ofaustenite grain at the time of heating and reducing grain size of therolled texture. In addition, fine TiN existing in a steel plate mayreduce the grain size of the weld heat-affected portion at the time ofwelding.

In order to obtain such an effect, at least about 0.005% of Ti can beused. However, excessive Ti can form TiC and may deterioratelow-temperature toughness and weldability. Therefore, the upper limit ofthe Ti content may be set to be about 0.025%.

Mg can depress the grain growth of austenite grains in the weldheat-affected portion and reduce the grain size. As a result, the weldportion may be given a high toughness. In order to realize such aneffect, it may be preferable for the exemplary steel to contain Mg ofabout 0.0002% or more. On the other hand, where the Mg content can beincreased, it may not be cost effective, because there may be lessenhancement of the effect of Mg compared with the increase of Mgcontent. Therefore, the upper limit of the Mg content can be set to beabout 0.0050%.

In addition to the above-described exemplary composition, the hightensile strength, refractory steel of an exemplary embodiment of thepresent invention can further include, in mass %, one or two selectedfrom Ca: about 0.0005 to 0.0040% and REM (Rare Earth Metal): about0.0005 to 0.0100%.

As the REM, one or more selected from rare earth metals such as Ce, La,and Nd or the like may be used.

Ca and REM may be effective in controlling MnS morphologies andimproving low-temperature toughness of the base metal. In addition, Caand REM may be effective in reducing sensitivity for hydrogen-inducedcracking, for example, hydrogen-induced cracking (HIC) under a humidhydrogen sulfide environment, SSC, and stress oriented HIC (SOHIC). Inorder to express such effects, at least a content of about 0.0005% canbe used.

However, if too much Ca or REM is contained, cleanability of steel maybe deteriorated, and toughness of the base metal and hydrogen inducedcracking (HIC, SSC, SOHIC) sensitivity under a humid hydrogen sulfideenvironment may be enhanced. Therefore, the upper limit of the Cacontent can be set to be about 0.0040% and the upper limit of the REMcontent may be set to be about 0.0100%. Since Ca and REM can express anearly similar effect, it is possible to load one of Ca and REM in theabove-described range, or it is possible to load a mixture of Ca and REMin the above-described exemplary range.

In the high tensile strength, refractory steel according to an exemplaryembodiment of the present invention, in order to ensure the yieldstrength of about 440 MPa or more, and yield strength at about 600° C.of not less than about ⅔ of the yield strength at room temperature, thatis, not less than about 294 MPa, while controlling Mo content to belower than about 0.70%, it can be preferable to inhibit or reduce acontrol of the exemplary steel composition also to control itsmicrostructure.

In the exemplary microstructure of the high tensile strength, refractorysteel according an exemplary embodiment of the present invention, anarea fraction of polygonal ferrite or pseudo polygonal ferrite in a ¼thick position in the plate thickness direction of a steel plate of thefinal rolling product may be controlled to be about 10% or less.

In the steel having the steel composition according to the exemplaryembodiment of the present invention in which the Mo content may berestricted to be lower than about 0.70%, especially in a thick steelplate thicker than about 40 mm, it may be difficult to ensure not onlythe room temperature strength, but also the high temperature strength ofthe steel, when the area fraction of polygonal ferrite or pseudopolygonal ferrite exceeds about 10%.

In the present invention, the microstructure is represented by thetexture on the plane along the final rolling direction, where the planeis in a ¼ thick position with respect to the section of the platethickness.

Although each component of the exemplary steel can be limited, it may bedifficult to obtain preferable properties if the component system as awhole is not appropriately controlled. Therefore, a weld crack sensitivecomposition PCM can be limited to be about 0.25% or less, where the PCMmay be defined by the following exemplary equation:

P_(CM)=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B.

The weld crack sensitive composition PCM can be a parameter indicatingweldability, and weldability may be satisfactory as PCM shows a lowvalue. In the steel according to the exemplary embodiment of the presentinvention, where the weld crack sensitive composition PCM may be 0.25%or less, it is possible to ensure excellent weldability while ensuringexcellent high temperature strength.

Further, a method for manufacturing a high tensile strength, refractorysteel according to an exemplary embodiment of the present invention isexplained.

The high tensile strength, refractory steel according to the presentinvention can be manufacture by the first or by the second exemplaryembodiment of a manufacturing method according to the present invention.

The first exemplary embodiment of the manufacturing method can include:heating a steel member in a form of billet or slab having the steelcomposition according to the exemplary embodiment of the presentinvention at a temperature of about 1100 to 1300° C.; rolling the steelmember at a temperature of about 800 to 950° C.; directly quenching thesteel member from a temperature of not lower than a higher one selectedfrom about 750° C. or a temperature about 150° C. lower than atemperature at a time of completing the rolling; and tempering the steelmember at a temperature not higher than Ac₁.

The second embodiment of the manufacturing method can include: hotrolling a steel member in a form of billet or slab having the steelcomposition according to the exemplary embodiment of the presentinvention; self-cooling the steel member: quenching the steel memberafter reheating the steel member at a temperature of about 900 to 950°C.; and tempering the steel member at a temperature not higher than Ac₁.

Firstly, the first exemplary embodiment of the manufacturing method asfollows. A steel member in a form of billet or slab having the steelcomposition according to the exemplary embodiment of the presentinvention can be heated at a temperature of about 1100 to 1300° C. Theexemplary reason for controlling the heating temperature preceding therolling to be about 1100 to 1300° may be to inhibit the coarsening ofaustenite grains to an unnecessary size in the time of heating, and torefine the rolled texture. The temperature of approximately 1300° C. canbe an upper limit of the temperature at which extreme coarsening ofaustenite is inhibited at the time of heating. Where the heatingtemperature exceeds the exemplary upper limit temperature, coarsegrained austenite may be mixed in the texture, and rolled austenitegrains also have a relatively large size. As a result, themetallographic structure after the phase transformation can berelatively coarse grained. In addition, in the transformation from thecoarse austenite, the microstructure may tend to become a bainiticstructure, possibly resulting in remarkable deterioration of toughnessof the exemplary steel. On the other hand, the lower limit of theheating temperature can be set to be about 1100° C. based on theconsideration of solution treatment of Nb so as to express the effect ofcontrolled rolling at the time of hot-rolling and precipitationhardening.

The exemplary steel member thus heated can be rolled at a temperature ofabout 800 to 950° C. The rolling temperature may be limited to be in therange of about 800 to 950° C. for the following exemplary reason. Wherethe rolling is performed at a temperature exceeding about 950° C., eventhough Mo and Nb are compositely loaded, grain size refining of therolled austenite may not be sufficient, and therefore low-temperaturetoughness may not be ensured stably even by performing subsequent directquenching and tempering. On the other hand, at a temperature lower thanabout 800° C., depending on plate thickness, precipitation of ferritecan occur before the direct quenching and may cause difficulty inensuring the microstructure, or Nb precipitates during rolling and doesnot contribute to the high temperature strength.

After completing the rolling, the steel member is directly quenched froma temperature of not lower than a higher one selected from 750° C. or atemperature 150° C. lower than a temperature at a time of completing therolling, that is a rolling finish temperature minus 150° C.

For example, the direct quenching temperature can be limited in theabove-described range. Firstly, in order to control the microstructurewith a purpose of ensuring the microstructure, the temperature can be atleast about 750° C. or more. On the other hand, even where thetemperature is not lower than about 750° C., when a temperature dropfrom the rolling finish temperature exceeds about 150° C., there may bea high possibility of recovery and recrystallization after the rollingor precipitation of Nb. In such a case, there is a possibility ofdeterioration of toughness or reduction of strength including hightemperature strength.

Therefore, the starting temperature of the direct quenching can belimited to be not lower than a higher one selected from about 750° C. ora temperature about 150° C. lower than a temperature at a time ofcompleting the rolling.

After the direct quenching, a tempering treatment can be performed at atemperature not higher than Ac₁.

In the exemplary embodiment of a steel member having a steel compositionaccording to the present invention, in certain cases, a temperature ofabout 700° C. or less is not higher than Ac₁. The practical treatmenttemperature can be set in accordance with target properties such asstrength.

Considering productivity and controllability of the heat treatmentfurnace in the industrial production, the preferable temperature of thetempering treatment may be about 450 to 650° C.

In the above-description, rolling temperature or the like can denote asurface temperature of the steel plate which may be monitored.

By the above-described exemplary method, high tensile strength,refractory steel according to an exemplary embodiment of the presentinvention can be manufactured.

According to the second exemplary embodiment of the manufacturingmethod, after hot rolling a steel member in a form of billet or slabhaving a steel composition according to the exemplary embodiment of thepresent invention, the steel member may be self-cooled. In thisexemplary case, the conditions of hot-rolling and self-cooling may notbe limited because the metallographic structure and material quality ofthe steel member is determined depending on the subsequent treatmentsincluding reheating, quenching and tempering.

Further, the hot-rolled and self-cooled steel member can be reheated ata temperature of about 900 to 950° C. and subjected to quenching.

It may be preferable to control the reheating and quenching temperatureto be higher than Ac₃ ( a temperature at which transformation of ferriteto austenite is completed at the time of heating) in terms ofmetallurgical definition.

In the steel member having a steel composition according to an exemplaryembodiment of the present invention, a temperature of about 900° C. ormore may be sufficient as the temperature not lower than Ac₃.

On the other hand, where reheating and quenching temperatures are toohigh, the metallic structure can be coarsened and low temperaturetoughness may be deteriorated. Therefore, the maximum temperature of thereheating and quenching can be set to be about 950° C.

In addition, the reheated and quenched steel member may be subjected totempering treatment at a temperature of not higher than Ac₁.

The conditions of the tempering treatment or the like may be similar orexactly the same as the above-described first exemplary embodiment ofthe manufacturing method.

By the above-described method, high tensile strength, refractory steelaccording to the exemplary embodiment of the present invention can bemanufactured.

The high tensile strength, refractory steel according to the exemplaryembodiment of the present invention can be applied to general weldconstruction steel not only for architectural construction but also forvarious applications including civil engineering, marine structures,ships and vessels, various storage tanks, or the like.

EXAMPLE

Further, high tensile strength, refractory steel having excellentweldability and gas cuttability according to an exemplary embodiment ofthe present invention is explained with reference to Examples 1 to 15and Comparative Examples 16 to 22.

Firstly, using a steel converter, steel slabs having variouscompositions shown in Table 1 were produced as ingots. Next, the slabswere subjected to various steel manufacturing processes using conditionsshown in Table 2, and steel plates each having a thickness (50 to 100mm) shown in Table 2 were manufactured.

Next, as shown in Table 2, each steel plate of Examples 1 to 15 andComparative Examples 16 to 22 was subjected to evaluation of the basemetal structure, mechanical properties, toughness of weld heat-affectedportion and roughness of gas-cut face.

In addition, as the mechanical properties, three parameters, that is,yield strength, tensile strength, and yield strength at 600° C. weremeasured, and yield ratio (yield strength/tensile strength (%)) wasdetermined from the yield strength and tensile strength. Then themechanical properties were evaluated.

With respect to the structure of the base metal, on a plane in a ¼ thickposition with respect to the section of the plate thickness, 10 fieldsof view were observed using a microscope with a magnification ratio of500. Thus, the area fraction (%) of polygonal ferrite (α_(p)) and areafraction of pseudo polygonal ferrite (α_(q)) were calculated.

With respect to yield strength and tensile strength, a test piece wassampled from a direction perpendicular to the rolling direction in thecentral portion of the plate thickness. Configuration of the test piecewas in accordance with a No.4 round bar for a test piece for testingtensile strength standardized by Japanese Industrial Standard JIS Z 2201“metallic material tensile strength test piece”. After that, yieldstrength and tensile strength were evaluated based on measurements inaccordance with Japanese Industrial Standard JIS Z 2241 “Method fortensile test of metallic material”.

For the evaluation of toughness of the base metal, a test piece wassampled from a direction perpendicular to the rolling direction in thecentral portion of the plate thickness. Configuration of the test piecewas in accordance with a 2 mmV notch impact test specimen standardizedby Japanese Industrial Standard JIS Z 2202 “impact test specimen ofmetallic material”. After that, toughness was evaluated based on themeasurement of fracture appearance transition temperature (vTrs (° C.))of the impact test specimen in accordance with Japanese IndustrialStandard JIS Z 2242 “Method for impacting test of metallic material”.

For the evaluation of toughness of the weld heat-affected portion, atest piece was sampled from a ¼ thick position of the plate thickness.Configuration of the test piece was in accordance with an impact testspecimen standardized by Japanese Industrial Standard JIS Z 2202 “impacttest specimen of metallic material”. Each test piece was subjected to aheat cycle corresponding to submerged arc welding (plate thickness 50mm) of energy input of 60 kJ/mm. The toughness was evaluated based onthe measurement of absorbed energy (vE₀) of the test piece at 0° C.

For the evaluation of roughness of gas-cut face, the highest height (Ry)of the surface roughness of the surface of each steel plate wasmeasured, where the definition of Ry was in accordance with JapaneseIndustrial Standard JIS B 0601 “Geometrical Property Standard (GPS) of aproduct-Surface Property: profile curve method-term, definition, andsurface property parameter”. Where the maximum height (Ry) was 50 μm orless, roughness was evaluated as satisfactory (A), and where the maximumheight (Ry) exceeded 50 μm, roughness was evaluated as unsatisfactory(B).

Target values of respective properties were 440 MPa for yield strength,−40° C. or less for fracture appearance transition temperature (vTrs),294MPa or more for yield strength at 600° C., and 100 J or more forabsorbed energy (vE₀) at 0° C.

Compositions of steels are shown in Table 1 and manufacturing processesof steel plates and various properties are shown in Table 2.

TABLE 1 Table CLASS OF COMPOSITION (mass %) STEEL C Si Mn P S Nb Mo Al NNi EXAMPLE 1 0.04 0.36 1.98 0.009 0.005 0.04 0.68 0.031 0.0042 2 0.040.45 1.80 0.012 0.004 0.05 0.64 0.035 0.0035 3 0.05 0.32 1.62 0.0100.003 0.04 0.60 0.003 0.0038 0.50 4 0.05 0.30 1.58 0.011 0.005 0.03 0.550.026 0.0032 5 0.06 0.24 1.55 0.008 0.006 0.01 0.58 0.042 0.0029 6 0.060.30 1.40 0.012 0.003 0.02 0.52 0.020 0.0034 0.14 7 0.07 0.35 1.46 0.0120.004 0.02 0.48 0.002 0.0048 0.35 8 0.07 0.30 1.35 0.011 0.006 0.02 0.370.033 0.0036 9 0.08 0.30 1.50 0.012 0.005 0.03 0.45 0.040 0.0035 10 0.080.56 1.52 0.010 0.005 0.01 0.40 0.055 0.0055 0.25 11 0.09 0.41 1.240.010 0.003 0.02 0.31 0.025 0.0038 0.20 12 0.09 0.35 0.80 0.011 0.0010.03 0.36 0.020 0.0064 13 0.10 0.25 0.85 0.012 0.004 0.02 0.45 0.0290.0040 14 0.12 0.33 0.55 0.010 0.003 0.02 0.50 0.019 0.0048 15 0.14 0.310.71 0.009 0.005 0.03 0.49 0.020 0.0078 COMPAR- 16 0.03 0.35 1.48 0.0120.005 0.04 0.50 0.025 0.0039 ATIVE 17 0.04 0.36 1.39 0.012 0.005 0.000.51 0.024 0.0041 18 0.04 0.08 0.45 0.014 0.004 0.03 0.60 0.026 0.00410.15 19 0.05 0.35 1.41 0.011 0.003 0.07 0.50 0.026 0.0038 20 0.15 0.431.18 0.010 0.006 0.02 0.49 0.030 0.0014 21 0.06 0.35 1.30 0.013 0.0080.02 0.25 0.029 0.0028 22 0.06 0.35 1.31 0.013 0.007 0.02 0.75 0.0280.0028 CLASS OF COMPOSITION (mass %) STEEL Cu Cr V B Ti Mg Ca P_(CM) *EXAMPLE 1 0.014 0.196 2 0.24 0.0016 0.200 3 0.028 0.012 0.193 4 0.176 50.009 0.184 6 0.14 0.38 0.203 7 0.34 0.010 0.0013 0.210 8 0.15 0.00080.184 9 0.195 10 0.19 0.015 0.215 11 0.25 0.010 0.202 12 0.12 0.0390.018 0.0014 0.176 13 0.044 0.185 14 0.005 0.192 15 0.010 0.0018 0.219COMPAR- 16 0.149 ATIVE 17 0.156 18 0.60 0.138 19 0.166 20 0.256 21 0.15322 0.187 * P_(CM) = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 +V/10 + 5B ** Underline denotes that the data is outside of the range ofthe present invention.

TABLE 2 TEMPER- TEMPER- ATURE ATURE QUENCH- HEATING AT AT ING TEMPERINGPLATE αp OR CLASS PRODUC- TEMPER- FINISHING STARTING TEMPER- TEMPER-THICK- αq OF TION ATURE ROLLING ROLLING ATURE ATURE NESS FRACTION STEELPROCESS * (° C.) (° C.) (° C.) (° C.) (° C.) (mm) (%) ** EXAMPLE 1 DQT1150 900 850 — 600 60 1 2 DQT 1100 810 760 — 580 50 2 3 QT 1200 — — 910550 80 0 4 DQT 1150 940 870 — 620 75 0 5 DQT 1150 870 800 — 600 60 0 6QT 1200 — — 930 580 50 0 7 QT 1250 — — 900 640 55 0 8 DQT 1200 920 880 —600 75 1 9 QT 1200 — — 920 500 100 0 10 DQT 1100 900 860 — 580 100 0 11DQT 1200 930 850 — 470 85 0 12 QT 1250 — — 910 520 80 0 13 QT 1200 — —940 600 75 0 14 QT 1250 — — 900 550 100 0 15 DQT 1250 900 860 — 620 75 0COMPAR- 16 DQT 1150 930 770 — 600 50 12 ATIVE 17 DQT 1100 850 730 — 60050 16 18 QT 1100 — — 900 600 50 3 19 AR 1200 860 — — — 75 79 20 QT 1250— — 900 600 75 0 21 QT 1200 — — 900 600 75 1 22 AR 1250 900 — — — 75 72TOUGHNESS TOUGH- OF ROUGH- YIELD NESS WELD-HEAT NESS CLASS YIELD TENSILEYIELD STRENGTH OF BASE AFFECTED OF OF STRENGTH STRENGTH RATIO AT 600° C.METAL PORTION GAS CUT STEEL (MPa) (MPa) (%) (MPa) vTrs(° C.) vE0(J)SURFACE EXAMPLE 1 520 623 83 345 −68 165 A 2 515 610 84 365 −72 171 A 3567 663 86 381 −64 154 A 4 482 581 83 344 −59 149 A 5 525 619 85 372 −62151 A 6 562 654 86 386 −57 160 A 7 543 628 86 370 −61 204 A 8 538 613 88368 −55 143 A 9 496 616 81 325 −51 153 A 10 509 602 85 336 −54 167 A 11485 619 78 317 −59 132 A 12 525 622 84 322 −53 129 A 13 533 614 87 371−50 152 A 14 468 593 79 316 −56 187 A 15 531 636 83 353 −47 137 ACOMPAR- 16 433 552 78 261 −62 174 A ATIVE 17 404 535 76 303 −58 168 A 18436 554 79 327 −51 155 A 19 313 468 67 235 −74  21 A 20 507 598 85 299−12  17 A 21 472 572 83 274 −52 104 A 22 324 491 66 243 −32  24 B * DQT:DIRECT QUENCHING - TEMPERING, QT: REHEATING QUENCHING-TEMPERING, AR: ASROLLED (AIR COOL) ** αp: polygonal ferrite, αq: pseudo polygonal ferrite*** Underline denotes that the data is outside of the range of thepresent invention.In accordance with the results of evaluation, all of Examples 1 to 15showed satisfactory properties.

On the other hand, compared with Examples 1 to 15, Comparative Examples16 to 22 having compositions out of the composition range of the presentinvention showed inferior values in fundamental properties such asstrength and toughness, and in high temperature strength, toughness ofweld-heat affected portion, gas cuttability or the like.

In Comparative Example 18, since the Ni content is lower than the Cucontent, cracks occurred during the hot-rolling, making it difficult toproduce the steel.

In Comparative Example 20, because of not only high C content but alsoof high PCM, root cracks were generated by an oblique y shape weld cracktest at room temperature.

According to the exemplary embodiment of the present invention, bycomposite loading of Nb while suppressing the content of Mo, a hightemperature strength of a high tensile strength steel having a yieldstrength of 440 MPa or more can be stably ensured. By suppressing thecontent of Mo, a deterioration of weldability and gas cuttability can belimited to a minimum level. At the same time, by limiting each of alloyelements such as C, Si, and Mn, as well as by limiting P_(CM), andfurther by limiting the microstructure of the steel and manufacturingconditions for same, composite properties such as excellent hightemperature strength, weldability, gas cuttability are compatiblyensured. Such high tensile strength, refractory steel having excellentweldability and gas cuttability can be widely applied as general weldconstruction steel for architectural constructions, civil engineering,marine structures, ships and vessels, various storage tanks or the like,and therefore has very large industrial applicability.

The foregoing merely illustrates the exemplary principles of the presentinvention. Various modifications and alterations to the describedembodiments will be apparent to those skilled in the art in view of theteachings herein. It will thus be appreciated that those skilled in theart will be able to devise numerous modification to the exemplaryembodiments of the present invention which, although not explicitlyshown or described herein, embody the principles of the invention andare thus within the spirit and scope of the invention. All publications,applications and patents cited above are incorporated herein byreference in their entireties.

1-16. (canceled)
 7. A high tensile strength, refractory steel having aparticular weldability and a particular gas cuttability, comprising: atleast one portion of a steel plate of a rolling product comprisingapproximately, in mass %, C: 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo:0.30% or more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to0.0060%, and a balance consisting of iron and unavoidable impurities,wherein a weld crack sensitive composition P_(CM) is about 0.25% orless, P_(CM) being defined as:P_(CM)=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B, and wherein anarea fraction of polygonal ferrite or pseudo polygonal ferrite in a ¼thick position in a plate thickness direction of the steel plate isabout 10% or less.
 8. The steel according to claim 7, wherein the atleast one portion further comprises approximately, in mass %, Ni: 0.05to 1.0%, Cu: 0.05 to 1.0%, and one or two or more selected from Cr: 0.05to 1.0%, V: 0.01 to 0.06%, B: 0.0002 to 0.0030%, Ti: 0.005 to 0.025%,and Mg: 0.0002 to 0.0050%, and wherein a content of Ni is equal to orgreater than half of a content of Cu.
 9. The steel according to claim 7,wherein the at least one portion further comprises approximately, inmass %, at least one of Ca: 0.0005 to 0.0040% or REM: 0.0005 to 0.0100%.10. The steel according to claim 7, wherein a yield strength of thesteel is about 440 MPa or more.
 11. A method for manufacturing a hightensile strength, refractory steel having a particular weldability and aparticular gas cuttability, comprising: heating a steel member at atemperature of about 1100 to 1300° C. in a form of at least one of abillet or a slab comprising approximately, in mass %, C: 0.04 to 0.14%,Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% orless, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%, Al:0.060% or less, N: 0.0010 to 0.0060%, and a balance consisting of ironand unavoidable impurities; rolling the steel member at a temperature ofabout 800 to 950° C.; directly quenching the steel member from atemperature greater or equal to a greater of about 750° C. or atemperature of about 150° C. lower than a temperature at a time ofcompleting the rolling; and tempering the steel member at a temperaturelower than or equal to Ac₁.
 12. A method for manufacturing a hightensile strength steel having a particular weldability and a particulargas cuttability, comprising: hot rolling a steel member in a form of atleast one of a billet or a slab, the steel member comprisingapproximately, in mass %, C: 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo:0.30% or more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to0.0060%, and a balance consisting of iron and unavoidable impurities;self-cooling the steel member: quenching the steel member afterreheating the steel member at a temperature of about 900 to 950° C.; andtempering the steel member at a temperature not higher than Ac₁.